Method of making a high frequency focused transducer

ABSTRACT

A high frequency focused transducer may be formed by fabricating a piezoelectric or ferroelectric wafer of a thickness less than about 100 microns and bonding a malleable sheet to the wafer with a thin layer of adhesive. Thereafter, the composite may be pressed into a spherical mold to form a curved transducer without fracturing the wafer. In another embodiment, a conductive adhesive layer may be applied to the wafer to a thickness sufficient to hold the wafer in a curved state, when set. After the adhesive is set, the composite may be pressed into the mold while the adhesive is held at an elevated temperature whereat it is elastic. Thereafter the composite is cooled so that the adhesive layer is stabilized and the curved transducer is removed from the well.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of making a high frequency focusedtransducer.

2. Description of the Related Art

Ultrasound imaging is now well established as an important medicaldiagnostic tool. This method relies on a device called a transducer tocreate a train of ultrasound pulses, in the frequency range from 2 to 10MHz, that are radiated into the body. Echoes returned from the tissuesbeing imaged are detected by the same transducer and transformed intoelectrical signals which can be displayed on a monitor. Image quality isprimarily determined by the ability of the transducer to focus theultrasound energy while the sensitivity of the imaging system isdetermined by the piezoelectric properties of the material that is usedin the transducer. The ultrasound transducer therefore plays a criticalrole in determining the performance of an imaging system. Current stateof the art single element transducers utilize spherically curvedradiators which are machined from a ceramic material such as LeadZirconate Titanate (PZT) or from a ceramic-polymer composite. PZT isusually the material of choice for clinical transducers because of itshigh efficiency and excellent electrical characteristics.

Recently, a number of new ultrasound imaging systems have been developedfor visualization of the eye, skin, endoluminal structures andintravascular structures at frequencies greater than 20 MHz.Unfortunately, at higher frequencies ceramic transducers are difficultto fabricate. In particular, obtaining a high frequency focusedtransducer is difficult since the thickness of the transducer material,being less than about one hundred microns, is too small for accuratemachining of the ceramic into a spherically shaped disk. Electronicfocusing using an array of elements is also difficult due to theprohibitively small element to element spacing that is required. Inlight of these problems, many high frequency imaging systems employ aplanar ultrasound transducer and either leave the beam unfocused orweakly focus the beam using a spherical reflector (mirror), both ofwhich degrade the lateral resolution of the system.

A method of fabricating spherically shaped 50-100 MHz transducers usinga piezoelectric polymer material (Poly(Vinylidene Fluoride)) is known.The flexibility of this polymer allows the fabrication of sphericallyfocused high frequency ultrasound transducers by deforming the materialabout a spherical object. Unfortunately, higher losses, and a lowerelectromechanical coupling coefficient make this type of transducerapproximately four to ten times less efficient than a ceramictransducer. Polymer transducers are also characterized by a lowdielectric constant which make it difficult to efficiently coupleelectrical energy to and from the transducer when the area of thetransducer is small. In spite of these disadvantages, their ease offabrication in spherical geometries have made polymer transducersdominant in applications at frequencies above 40 MHz.

This invention seeks to overcome drawbacks of known prior art highfrequency focused transducers.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a method offabricating a wafer assembly for a high frequency focused transducercomprising the following steps: fabricating a piezoelectric orferroelectric wafer of a thickness of less than about one hundredmicrons, said thickness chosen to provide resonance at a desired highfrequency of greater than 20 MHz; adhesively bonding a conductivemalleable sheet over a back face of said wafer whereby said adhesivestabilises said wafer, said malleable sheet being chosen to havesufficient strength to hold said wafer in a curved state; pressing saidwafer with said malleable sheet into a mold in order to form a curve insaid wafer and said malleable sheet.

In accordance with a further embodiment of the present invention, thereis provided a method of fabricating a wafer assembly for a highfrequency focused transducer comprising the following steps: fabricatinga piezoelectric or ferroelectric wafer of a thickness of less than aboutone hundred microns, said thickness chosen to provide resonance at adesired high frequency of greater than 20 MHz; applying a conductiveadhesive layer over back face of said wafer comprised of an adhesive ofthe type which, when set, is elastic over a first range of temperaturesand is stable over a second lower range of temperatures, said secondrange of temperature including the temperatures at which said transducerwill operate, said conductive adhesive layer being applied to thicknessof about one hundred microns such that said adhesive layer is sufficientto hold said wafer in a curved state, when said adhesive layer is set;while said adhesive is within said first range of temperatures, pressingsaid wafer with said conductive adhesive layer into a mold in order toform a curve in said wafer; and cooling said wafer with said conductiveadhesive layer to said second range of temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures which disclose example embodiments of the invention,

FIG. 1 is an enlarged schematic cross-sectional side view of a portionof a transducer made in accordance with one embodiment of thisinvention,

FIG. 2 an enlarged schematic cross-sectional side view of a portion of atransducer made in accordance with another embodiment of this invention,and

FIG. 3 is a partially cross-sectional side view of a transducer assemblyincorporating a transducer made in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, a transducer 10 made in accordance with thisinvention comprises a wafer 12 having a thickness T of less than aboutone hundred microns. The wafer is fabricated of a piezoelectric orferroelectric material, such as lithium niobate or lead zirconatetitanate (PZT) and has a thin electrode layer 15, 17 deposited on itsback 16 and front 24 faces, respectively. The electrode layers may beabout one thousand Angstroms thick. A thin adhesive layer 18 of a fewmicrons in thickness is applied over the back face 16 of the wafer 12. Aconductive malleable sheet, namely, metal sheet 20 is bonded by theadhesive layer 18 to face 16.

As will be apparent from FIG. 1, the wafer 12 is curved along itssurface 16. This curvature of the wafer is what focuses the transducer10.

Wafers made of a piezoelectric or ferroelectric material are brittle andwill normally fracture rather than deform under the influence of anapplied force. The subject invention, however, allows a deforming forceto be applied to the very thin wafer 12 without fracturing the waferthus allowing the wafer to be curved so that it may be fabricated into ahigh frequency focused transducer.

The steps involved in the fabrication high frequency transducer 10 ofthis invention are as follows. A bulk sample of a piezoelectric orferroelectric material is lapped to a wafer thickness corresponding tothe desired resonant frequency. For example, for a fifty MHz PZTtransducer, the thickness of the wafer should be approximately fortymicrons. The electrode layers 15, 17 are next deposited onto the wafer.A thin adhesive layer 18 is then applied to the back face 16 of thewafer. Where the wafer is made of a ceramic (typically PZT), theadhesive tends to penetrate the fissures between the grains. A metalsheet 20 is applied to the thin adhesive layer 18 and the adhesive isset to bond the metal layer 20 to the wafer 12. The composite may thenbe cut to its finished dimensions.

The composite wafer 12 with metal sheet 20 bonded thereto by the thinadhesive layer may be pressed into a mold comprising a well with aconcave spherically shaped surface using a ball bearing. Thisspherically curves the wafer along face 16 in order to focus thetransducer. By bonding the metal backing layer 20 onto the wafer,stresses which are created when the transducer is pressed into the wellare evenly distributed across the surface of the material. Localconcentrations of stress which lead to fracture are avoided so that thewafer may be curved without cracking. In addition, particularly wherethe wafer is ceramic, the penetration of the adhesive into the fissuresof the wafer further stabilises the wafer to assist in avoidingcracking.

Because of the thinness of the adhesive layer 18, the metal backinglayer 20 will make contact with the electrode layer 16 at a number ofpoints when the composite is deformed in the mold. This ensures goodelectrical contact between the metal layer and the electrode layer.Optionally, the adhesive layer 18 may be comprised of a conductiveadhesive which will provide uniform electrical contact between theelectrode layer and the metal layer.

A wide range of adhesives are suitable for layer 18.

With reference to FIG. 2 wherein like reference numerals indicate likeparts, a transducer 100 made in accordance with a second embodiment ofthe subject invention comprises a wafer 12 with a conductive adhesivelayer 118 adhered to the electrode layer 15 of the back face 16 of thewafer. The conductive adhesive layer has a thickness of at least aboutone hundred microns which is sufficient to hold wafer 12 in a curvedshape without the need for a malleable backing sheet. A thick conductiveadhesive backing layer 122 of about one millimeter in thickness is castonto the back of conductive adhesive layer 118. Wafer 12 is curved alongits face 16.

The steps involved in fabricating transducer 100 are as follows.Firstly, as before, a piezoelectric or ferroelectric material is lappedto a wafer thickness corresponding to the desired resonant frequency andelectrode layers 15, 17 are deposited on the front and back surfaces ofthe wafer. Conductive adhesive layer 118 of about one hundred microns inthickness is next applied over the back face 16 of the wafer 12 and set.The composite may then be cut to its finished dimensions.

The adhesive chosen for the conductive adhesive layer 118 must be onewhich is stable (i.e., not elastic) at the operating temperature rangefor the transducer 100 (normally room temperature) and which is elasticat an elevated temperature. By meeting these criteria an adhesive may bemade to emulate a malleable material. An epoxy resin meets thesecriteria and is the preferred choice for the adhesive of this layer.

The composite wafer with adhesive layer is heated sufficiently so thatthe adhesive of layer 118 is elastic and the composite is then pressedin the mold. Stresses which are created when the transducer is pressedinto the well are evenly distributed across the surface of the wafer bythe adhesive layer 118. Once again, particularly where the wafer isceramic, the penetration of the adhesive into the fissures of the waferfurther stabilises the wafer to assist in avoiding cracking. After thecomposite wafer with adhesive layer has been deformed, the curvedcomposite is cooled so that the adhesive is stabilised and the compositemay then be removed from the mold. The stabilised adhesive layer 118will retain the wafer in its curved shape provided it has a sufficientthickness. It has been found that a thickness of at least about onehundred microns for the adhesive layer is sufficient.

A relatively thick (one millimeter) adhesive backing layer 122 is nextcast onto the back surface of the adhesive layer 118. The backing layer122 provides a rigid support for the composite.

The curved wafer 12 with layers 18 and 20 (FIG. 1) or layers 118 and 122(FIG. 2) may then be mounted in a barrel connector 230 shown in FIG. 3.Centre pin 232 of the barrel connector makes electrical contact with theconductive metal sheet 20 (FIG. 1) or conductive adhesive layer 118(FIG. 2). A plastic insert 234 positions and holds the wafer 12 (FIG. 1and 2) with its layers within the barrel connector 230. After assemblyinto the barrel connector 230, an conductive electrode layer 26 may beevaporated across the front face 24 (FIGS. 1 and 2) of the wafer 12,plastic insert 234, and housing 238 of the connector 230 in order tocomplete transducer 10 (of FIG. 1) or 100 (of FIG. 2). Housing 238 ismade of a conducting metal so that an electrical potential may beapplied between housing 238 and center pin 232 in order to apply apotential across the transducer. The resulting focused ultrasonic waveswhich emanate from the transducer 100 are indicated at 240 with thefocus indicated at 242.

The high frequency transducer of FIG. 1 or 2 may be modified byadditionally bonding a material 26 over the front surface 24 of thewafer 12.

The manufacturing technique of this invention is not effective for a lowfrequency transducer since, due to the thickness of the wafer, theadhesive will not sufficiently stabilise the wafer in order to allow itto be deformed in a mold. Those skilled in the art will realise thateven with the technique of the present invention, there are limits tothe amount the wafer may be curved. However, the invention permitssufficient curvature of a wafer to provide a focused transducer.

Other modification will be apparent to those skilled in the art and,accordingly, the invention is defined in the claims.

What is claimed is:
 1. A method of fabricating a wafer assembly for ahigh frequency focused transducer comprising the followingsteps:fabricating a piezoelectric or ferroelectric wafer of a thicknessof less than about one hundred microns, said thickness chosen to provideresonance at a desired high frequency of greater than 20 MHz; adhesivelybonding a conductive malleable sheet over a back face of said waferwhereby said adhesive stabilises said wafer, said malleable sheet beingchosen to have sufficient strength to hold said wafer in a curved state;pressing said wafer with said malleable sheet into a mold in order toform a curve in said wafer and said malleable sheet.
 2. The method ofclaim 1 including the step of removing said curved wafer with malleablesheet from said mold after said pressing step.
 3. The method of claim 1wherein the step of adhesively bonding a conductive malleable sheet to aback face of said wafer comprises bonding a malleable sheet to said backface of said wafer with an adhesive layer having a thickness of a fewmicrons.
 4. The method of claim 3 wherein the step of bonding amalleable sheet to said back face of said wafer with an adhesive layercomprises utilising a conductive adhesive for said adhesive layer. 5.The method of claim 3 including the step of applying an electrode over afront face of said wafer.
 6. The method of claim 1 wherein said step offabricating a piezoelectric or ferroelectric wafer comprises choosing amaterial for said wafer selected from the class of piezoelectriccrystals and piezoelectric ceramics.
 7. A method of fabricating a waferassembly for a high frequency focused transducer comprising thefollowing steps:fabricating a piezoelectric or ferroelectric wafer of athickness of less than about one hundred microns, said thickness chosento provide resonance at a desired high frequency of greater than 20 MHz;applying a conductive adhesive layer over a back face of said wafercomprised of an adhesive of the type which, when set, is elastic over afirst range of temperatures and is stable over a second lower range oftemperatures, said second range of temperatures including thetemperatures at which said transducer will operate, said conductiveadhesive layer being applied to a thickness of about one hundred micronssuch that said adhesive layer is sufficient to hold said wafer in acurved state, when said adhesive layer is set; while said adhesive iswithin said first range of temperatures, pressing said wafer with saidconductive adhesive layer into a mold in order to form a curve in saidwafer; and cooling said wafer with said conductive adhesive layer tosaid second range of temperatures while said wafer with said conductivelayer is pressed in said mold.
 8. The method of claim 7 wherein saidstep of fabricating a piezoelectric or ferroelectric wafer compriseschoosing a material for said wafer selected from the class ofpiezoelectric crystals and piezoelectric ceramics.
 9. The method ofclaim 7 wherein the step of applying a conductive adhesive layercomprises applying a conductive epoxy resin.
 10. The method of claim 7including the step of removing said curved wafer with conductiveadhesive layer from said mold after said cooling step.
 11. The method ofclaim 10 including the step of applying an electrode over a front faceof said wafer.
 12. The method of claim 10 including the step of applyinga backing conductive adhesive layer having a thickness of about onemillimeter to said conductive adhesive layer after said step of removingsaid curved wafer with adhesive layer from said mold.
 13. A method offabricating a wafer assembly for a high frequency focused transducercomprising the following steps:fabricating a piezoelectric orferroelectric wafer to a thickness of less than one hundred microns froma material selected from the class of piezoelectric crystals andpiezoelectric ceramics; adhesively bonding a conductive malleable metalsheet over a back face of said wafer with an adhesive layer having athickness of a few microns whereby said adhesive stabilises said wafer,said malleable metal sheet being chosen to have sufficient strength tohold said wafer in a curved state; pressing said wafer with saidmalleable metal sheet into a well having a spherical surface in order toform a spherical curve in said wafer and said malleable metal sheet; andremoving said curved wafer with malleable metal sheet from said well.14. A method of fabricating a wafer assembly for a high frequencyfocused transducer comprising the following steps:fabricating apiezoelectric or ferroelectric wafer to a thickness of less than onehundred microns from a material selected from the class of piezoelectriccrystals and piezoelectric ceramics; applying a conductive epoxy resinlayer over a back face of said wafer such that said epoxy resin layerwhen set, is elastic over a first range of temperatures and is stableover a second lower range of temperatures, said second range oftemperatures including the temperatures at which said transducer willoperate, said conductive epoxy resin layer being applied to a thicknessof about one hundred microns; while said epoxy resin layer is withinsaid first range of temperatures, pressing said wafer with saidconductive epoxy resin layer into a well having a spherical surface inorder to form a spherical curve in said wafer; cooling said wafer withsaid conductive epoxy resin layer to said second range of temperatures;and removing said curved wafer with conductive epoxy resin layer fromsaid well after said cooling step.